BIRD OF PREY - ANYONE KNOW WHAT THIS IS?!

I found this little beauty in the Mendips in Somerset. Really not sure what it could be, it looks smallish and it's chest looks nearly all white (not streaked - as per the sparrowhawk for example). The photo was taken in August last year. It's landed on the back of an unfortunate pigeon. Sorry about the quality of the images...it was taken from my iphone inside a car...

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